The present invention relates to a glass sheet with a conductive film, a method of manufacturing the same, and a photoelectric conversion device, such as a solar cell or the like, using the same.
Generally, a thin film photoelectric conversion device includes a transparent conductive film containing tin oxide as a main component, a thin film silicon layer as a photovoltaic layer, and a back electrode of aluminum or the like, which are formed on a glass substrate in this order. In such a configuration, the transparent conductive film positioned on a window side from which light is introduced into the photovoltaic layer is required to have high light transmittance. It also is required to have high conductivity (low resistance) in view of its function as an electrode. Furthermore, it has been known that when the transparent conductive film is formed to have a surface with roughness, the introduced light can be used efficiently due to a so-called xe2x80x9clight trapping effectxe2x80x9d. The degree of the roughness formed at the surface of the transparent conductive film can be indicated by a haze ratio.
As the transparent conductive film, a fluorine doped tin oxide (hereinafter referred to as xe2x80x9cSnO2:Fxe2x80x9d) film has been used in many cases. This film is more excellent in chemical stability such as plasma resistance or the like compared to a tin doped indium oxide (ITO) film. Therefore, the SnO2:F film does not deteriorate greatly in forming a photovoltaic layer (a thin film silicon layer) by using a plasma CVD method. The SnO2:F film is formed to have a certain thickness in order to decrease its sheet resistance to a value that is desirable for an electrode, although its sheet resistance is intended to be decreased by doping with fluorine. Therefore, in order to improve the conversion efficiency of the photoelectric conversion device, it is necessary to improve the light transmittance in view of the thickness of the SnO2:F film.
For instance, JP 1-259572 A describes that the light absorption of a transparent conductive film is suppressed by setting the concentration of chlorine introduced into a SnO2:F film from raw materials in forming the film to be 0.40 wt. % or lower. The specific chlorine concentration described therein is 0.12 to 0.36 wt. %. A glass sheet used therein is a soda-lime glass sheet precut to have a predetermined size.
JP 2-181473 A describes that the plasma resistance of a SnO2:F film is improved by adjusting levels of trace components in the SnO2:F film. JP 2-181473 A discloses that fluorine in the film deteriorates the plasma resistance of the SnO2:F film more than the chlorine in the film does. JP 2-181473 A also describes a SnO2:F film in which the fluorine concentration is 0.04 to 0.2 wt. % and the chlorine concentration is 0.4 wt. % or lower. In a film with a low chlorine concentration, the fluorine concentration is adjusted to be further lower than the chlorine concentration. Further, as a heating temperature of a glass sheet, a temperature of 550xc2x0 C. is described as an example. The glass sheet used therein also is a soda-lime glass sheet precut to have a predetermined size.
However, the influences of film formation conditions and trace components other than chlorine on the light transmittance of a conductive film such as a SnO2:F film have not been clarified.
Further, the following problem has been pointed out. In a thin film photoelectric conversion device, defect levels are formed in an amorphous silicon-based photovoltaic layer due to light irradiation, thus decreasing a photoelectromotive force. Recently, therefore, an amorphous silicon-based photovoltaic layer tends to be formed to be thin. However, in the above-mentioned conventional SnO2:F film, components other than chlorine and characteristics of the film have not been adjusted to satisfy the characteristics required for the silicon-based photovoltaic layer with a reduced thickness.
Further, the light trapping effect by a transparent conductive film becomes more important in a thinner photovoltaic layer. Therefore, the transparent conductive film is required to have a high haze ratio in addition to a low absorptance for achieving a high light transmittance. In order to increase the haze ratio, the thickness of the film may be increased.
However, the simple increase in film thickness results in the increase in absorption of the film.
The absorptance and the haze ratio of the SnO2:F film are affected by manufacturing conditions and raw materials. One of the typical raw materials used for forming a tin oxide film by a CVD method is tetramethyltin. For instance, JP 55-56041 A discloses a method of forming a tin oxide film by pyrolysis of a mixed gas containing tetramethyltin and oxygen. This mixed gas contains about 1% tetramethyltin and about 20% oxygen. Since tetramethyltin has a high reactivity, the control of film formation is not easy.
JP 2-175631 A and JP 8-508006 A disclose a method of forming a tin oxide film with a mixed gas containing tin tetrachloride and dry air. Since tin tetrachloride has a high reactivity with respect to water, the dry air is used as a gas for oxidizing the tin tetrachloride. Further, JP56-24708 A describes that hydrogen is added as a reductant for suppressing the reaction between tin tetrachloride and water.
Materials that can be handled more easily than those tin materials include organic tin chlorides such as dimethyltin dichloride, monobutyltin trichloride, and the like. For instance, JP 4-502305 A discloses a method of forming a film using dimethyltin dichloride and describes a mixed gas containing dimethyltin dichloride, oxygen, water, and nitrogen. This mixed gas is indicated to have a volume ratio of dimethyltin dichloride:oxygen:water:nitrogen=50:50:23:250.
JP 63-242947 A describes an example in which the haze ratio of a tin oxide film is varied by adjusting the mole ratio of dimethyltin dichloride and water. Oxygen is not added simultaneously.
U.S. Pat. No. 5,698,262 describes a method of doping a tin oxide film with fluorine using not trifluoroacetic acid or the like but hydrogen fluoride in order to obtain low and uniform sheet resistance. The mixed gas disclosed therein contains dimethyltin dichloride, oxygen, water, hydrogen fluoride, and helium. The concentrations of oxygen and water vapor are 10 to 60 mol % and 2 to 50 mol %, respectively. In the examples and comparative examples, the concentration of dimethyltin dichloride (vapor) is described as about 2.5 mol %. However, the thickness (320 nm) of the SnO2:F film disclosed therein limits the size of crystal grains, thus preventing the film surface from having great irregularities. Therefore, a high haze ratio cannot be obtained.
The present invention is intended to provide a glass sheet with a conductive film having a higher light transmittance than that in a conventional one and a method of manufacturing the same, by directing attention not only to chlorine but also to trace components other than that.
Another object of the present invention is to provide a glass sheet with a conductive film that can be used suitably even when a photoelectric conversion layer is thin. Particularly, the present invention is intended to provide a glass sheet with a conductive film having both a low light absorptance and a high haze ratio and a method of manufacturing the same.
Furthermore, still another object of the present invention is to provide a photoelectric conversion device such as a photovoltaic device using any one of the above-mentioned glass sheets.
The present inventors found that especially the concentrations of fluorine and carbon in a conductive film containing tin oxide as a main component affected the light transmittance of the conductive film. The present inventors also found that an oxygen concentration in the atmosphere in forming the conductive film affected the light transmittance. By controlling these conditions suitably, the absorption coefficient of the conductive film containing tin oxide as the main component can be decreased compared to that of a conventional one.
A first glass sheet with a conductive film of the present invention includes a glass sheet and a conductive film containing tin oxide as a main component formed on the glass sheet. In the first glass sheet with a conductive film, the conductive film has an absorption coefficient of 1.2xc3x97103 cmxe2x88x921 or less in a wavelength region between 400 nm and 1100 nm.
In the present specification, the absorption coefficient denotes a coefficient k (cmxe2x88x921) in a relative equation of I=I0xc2x7exe2x88x92kd, by which when incident light with an intensity I0 that has entered a thin film travels in its thickness direction for a distance d(cm) to have an intensity I, I and I0 are expressed.
It is preferable that the absorption coefficient of the conductive film is 0.7xc3x97103 cmxe2x88x921 or less in the wavelength region between 500 nm and 900 nm, particularly 0.7xc3x97103 cmxe2x88x921 or less in a wider wavelength region between 400 nm and 900 nm.
Preferably, the fluorine concentration in the conductive film is 0.1 wt. % or lower, further preferably 0.08 wt. % or lower, since an excessively high fluorine concentration results in an excessively high absorption coefficient of the conductive film. On the other hand, it is preferable that the fluorine concentration in the conductive film is at least 0.03 wt. %, further preferably at least 0.05 wt. %, since an excessively low fluorine concentration results in an excessively low specific resistance of the conductive film.
When the carbon concentration in the conductive film is expressed by a ratio of carbon atoms to tin atoms (a C/Sn atom ratio), it is preferable that the ratio is 0.015 or less, more preferably 0.010 or less, and particularly preferably 0.007 or less, since an excessively high carbon concentration results in an excessively high absorption coefficient of the conductive film.
It is preferable that the conductive film is formed by a pyrolytic oxidation reaction in an atmosphere with an oxygen concentration of at least 10 vol. %. The reason is that it is disadvantageous in decreasing the absorption coefficient of the conductive film if carbon contained in the tin material is not decomposed and thus remains in the film.
A first method of manufacturing a glass sheet with a conductive film of the present invention is characterized as follows. By a pyrolytic oxidation reaction in an atmosphere with an oxygen concentration of at least 10 vol. %, a conductive film containing tin oxide as the main component and having an absorption coefficient of 1.2xc3x97103 cmxe2x88x921 or less in the wavelength region between 400 nm and 1100 nm is formed on glass, specifically on a glass sheet or a glass ribbon during manufacture of a glass sheet.
In the manufacturing method, it is preferable that the conductive film is formed by a pyrolytic oxidation reaction in an atmosphere including oxygen with a volume ratio of at least 15 vol %. According to this preferable example, the absorption coefficient of the conductive film can be suppressed to be 0.7xc3x97103 cmxe2x88x921 or less in the wavelength region between 500 nm and 900 nm.
In the manufacturing method, it is preferred to form the conductive film on a glass sheet or a glass ribbon having a temperature of at least 590xc2x0 C. by the pyrolytic oxidation reaction.
In the glass sheet with a conductive film according to the present invention, it is preferred to form the conductive film via an undercoating film on a glass sheet. When an alkaline component contained in the glass sheet diffuses in the conductive film, characteristics such as conductivity or the like of the conductive film may be deteriorated in some cases. When the undercoating film is formed, diffusion of the alkaline component from the glass sheet into the conductive film can be suppressed. Therefore, the formation of the undercoating film is advantageous in obtaining the compatibility between a high light transmittance and a high conductivity in the conductive film.
The present invention also provides a glass sheet with a conductive film that can be used suitably even when a photoelectric conversion layer is thin.
A second glass sheet with a conductive film includes a glass sheet and a conductive film containing tin oxide as a main component formed on the glass sheet. In the second glass sheet with a conductive film, the chlorine concentration in the conductive film is 0.11 wt. % or lower and the fluorine concentration in the conductive film is equal to or higher than the chlorine concentration.
A third glass sheet with a conductive film of the present invention includes a glass sheet and a conductive film containing tin oxide as a main component formed on the glass sheet. In the third glass sheet with a conductive film, the chlorine concentration in the conductive film is 0.11 wt. % or lower and the fluorine concentration in the conductive film is at least 0.05 wt. %
According to each glass sheet with a conductive film described above, the conductive film is formed so that the glass sheet with a conductive film has a light transmittance of at least 79%, preferably at least 80%, with respect to incident light with a wavelength of 550 nm entering from the glass sheet side when the glass sheet is a soda-lime silica glass sheet with a thickness of 5 mm. In this case, the light transmittance denotes a total transmittance. While the glass sheet with a conductive film has such a high light transmittance, at least a predetermined amount of fluorine is contained in the conductive film. Therefore, the sheet resistance of the film can be decreased easily to be in a preferable range.
Specifically, it is preferable that the glass sheet with a conductive film has a light transmittance of at least 79% and is provided with a conductive film having a sheet resistance of 20 xcexa9/sq. (xcexa9/xe2x96xa1) or lower.
In the glass sheet with a conductive film, it is preferable that the conductive film is formed so that the glass sheet with a conductive film has a light transmittance of at least 79% with respect to incident light with a wavelength of 550 nm entering from the glass sheet side and the haze ratio in the wavelength is at least 4% when the glass sheet is a soda-lime silica glass sheet with a thickness of 5 mm. According to this preferable example, even when the thickness of the photoelectric conversion layer is reduced, light can be used efficiently due to the xe2x80x9clight trapping effectxe2x80x9d.
A fourth glass sheet with a conductive film of the present invention for achieving such efficient use of light includes a glass sheet and a conductive film containing tin oxide as a main component formed on the glass sheet. In the fourth glass sheet with a conductive film, the chlorine concentration in the conductive film is 0.11 wt. % or lower. Further, the conductive film is formed so that the glass sheet with a conductive film has a light transmittance of at least 79%, preferably at least 80%, with respect to incident light with a wavelength of 550 nm entering from the glass sheet side and the haze ratio in the wavelength is at least 4% when the glass sheet is a soda-lime silica glass sheet with a thickness of 5 mm. This substrate may be used particularly for a photoelectric conversion device with a thinner photoelectric conversion layer.
In the above-mentioned glass sheet with a conductive film, it also is preferred to form the conductive film on an undercoating film on the glass sheet. When the undercoating film is formed, diffusion of an alkaline component contained in the glass sheet into the conductive film can be suppressed. In addition, when the undercoating film is utilized as an anti-reflection film, the light transmittance of the glass sheet with a conductive film can be improved further.
Further, the present inventors found that when a relatively thick conductive film (with a thickness of at least 400 nm) was formed, in order to obtain the compatibility between a low light absorptance and a high haze ratio, the ratio of a total amount of oxygen and water vapor to tin atoms in a mixed gas was important in view of the pyrolytic reaction of the mixed gas.
A second method of manufacturing a glass sheet with a conductive film is one for manufacturing a glass sheet with a conductive film in which a conductive film having a thickness of at least 400 nm and containing tin oxide as a main component is formed on a glass sheet. In the second method, the conductive film is formed on glass, specifically, on a glass sheet or a glass ribbon during manufacture of a glass sheet by thermally decomposing (pyrolyzing) a mixed gas containing vapor of an organic tin compound, oxygen, and water vapor wherein a total amount of oxygen (O2) and water vapor (H2O) is at least 14 moles to one mole of tin atoms (Sn) contained in the organic tin compound.
In the mixed gas, the volume ratio (mole ratio) of the total amount of oxygen and water vapor to the vapor of an organic tin compound is at least 14 as long as the condition that one tin atom is contained per molecule of the organic tin compound is satisfied. For simplification, in the following description, considering that only one tin atom is contained per molecule of a generally used organic tin compound, the concentrations of gas components such as oxygen and the like are described using volume ratios of respective gas components to the vapor of an organic tin compound (for example, indicated as O2/Sn) on the premise that the above condition is satisfied.
A fifth glass sheet with a conductive film of the present invention is manufactured by the above-mentioned manufacturing method. In this glass sheet with a conductive film, a conductive film with a thickness of at least 400 nm containing tin oxide as a main component is formed on a glass sheet. The glass sheet is obtained by forming, on glass, the conductive film with a mean absorptance of 4.5% or less in the wavelength region between 400 nm and 800 nm by thermally decomposing a mixed gas containing vapor of an organic tin compound, oxygen, and water vapor, wherein a total amount of oxygen and water vapor is at least 14 moles to one mole of tin atoms contained in the organic tin compound.
It is preferable that the glass sheet with a conductive film has a haze ratio of at least 2.0% in a wavelength of 550 nm.
The present invention also provides a photoelectric conversion device using one of the above-mentioned glass sheets with a conductive film. In this photoelectric conversion device, at least one photoelectric conversion unit and a back electrode are formed in this order on a conductive film of the glass sheet with a conductive film (the glass sheets with a conductive film obtained according to the above-mentioned manufacturing methods also can be used). This photoelectric conversion device is used with the glass sheet side positioned as the light incident side.